A principled reaction, played after a long reflection. One feature of Ding's style is that he's not afraid to go for complicated and forcing variations, even when he might face advanced preparation.
5. f1g2  5... g8f6  6. e1g1  6... e8g8 The system with c6-d5 in the Grunfeld Fianchetto has been played in thousands of games, even at the world championship level. It's one of Black's most solid options. 7. f1e1 A big surprise. I've never seen this move before. Can you imagine this is only the 15th most-popular move in the position $2 It was played by a GM just twice before, both times in blitz games. So it's safe to say that this is nearly a new idea.